A vehicle is conventionally provided with an engine control apparatus which is adapted to perform such an air-fuel ratio feedback control as to determine a fuel injection quantity to an engine so that an air-fuel ratio of an air-fuel mixture be controlled to a target value based upon an output signal of an air-fuel ratio sensor disposed in an exhaust system of the engine (e.g., JP 2000-241381A).
In addition, a concentration cell oxygen sensor is known as an air-fuel ratio sensor. This concentration cell oxygen sensor is, as exemplified in FIG. 11, constructed in such a manner that electrodes 103 and 105 made of platinum or the like are located respectively on the outer surface and the inner surface of a cup-shaped solid electrolyte 101 made of zirconia or the like. The solid electrolyte 101 and the electrodes 103, 105 form a detecting element. The outer side of the cup-shaped solid electrolyte 101 is exposed to exhaust gases and air is introduced inside the cup-shaped solid electrolyte 101. Accordingly, the electrode 103 located on the outer surface of the solid electrolyte 101 serves as an exhaust-side electrode facing exhaust gases and the electrode 105 located on the inner surface of the solid electrolyte 101 serves as an atmosphere-side electrode facing an atmosphere.
In such a concentration cell oxygen sensor, a density difference between the oxygen density of the exhaust-side electrode 103 and the oxygen density of the atmosphere-side electrode 105 produces an electromotive force and a potential difference between the electrodes is detected as an output voltage of the oxygen sensor. The output voltage changes sharply or in stepwise in the vicinity of a stoichiometric air-fuel ratio. In a range richer in fuel than the stoichiometric air-fuel ratio, the output voltage becomes 1 V and in a range leaner in fuel than the stoichiometric air-fuel ratio, the output voltage becomes about 0 V (e.g., JP-2000-241381A). In this case, electrons flow from the exhaust-side electrode 103 to the atmosphere-side electrode 105 to produce the electromotive force. Therefore, the output voltage can be produced as a potential difference on the basis of the exhaust-side electrode 103 as a reference.
The solid electrolyte 101 becomes in an activated state in which the solid electrolyte 101 serves as an oxygen ion conductor at a temperature more than a certain activation temperature (e.g., 300° C.) and produces an electromotive force as a concentration cell in response to a difference in oxygen density between the inner surface (atmosphere-side face) and the outer surface (exhaust-side face) of the solid electrolyte 101. Therefore, the solid electrolyte 101 is heated to the activation temperature by a heater so that it may become operative at earlier time. Even if the solid electrolyte 101 is thus activated, this oxygen sensor will not operate if the sensor element including the solid electrolyte 101 cracks.